For example, axial steam turbines used for power generation and the like include a plurality of turbine stator cascades and turbine rotor cascades. The turbine stator cascades each include a plurality of turbine stator blades, whereas the turbine rotor cascades each include a plurality of turbine rotor blades.
An integral shroud blade, disclosed as the turbine rotor blade in Japanese Patent Application Publication No. H4-5402, has an integral shroud at the distal end thereof. The blade is configured to deform by centrifugal force so as to be in a detorsion state when a turbine is operated, so that adjacent integral shrouds come into contact with each other.
In some cases, the turbine rotor blade of this type has an intermediate portion provided with an integral stub, as described in Japanese Patent Application Publication No. H4-5402, for achieving higher structural damping as a measure mainly focusing on an increase in stress due to an increase in a length of a steam turbine long blade. The integral shroud blade disclosed in Japanese Patent Application Publication No. H4-5402 is provided with the integral shroud on the distal end portion, and is further provided with integral stubs, as triangular protrusions, on both side surfaces at the intermediate portion of the blade. When the detorsion of the blade is achieved by the centrifugal force, the adjacent integral stubs come into contact with each other, whereby the higher structural damping is achieved.
Japanese Patent No. 4058906 and Japanese Patent Application Publication No. 2011-137424 each also disclose a turbine rotor blade with a distal end portion provided with an integral shroud, and with an intermediate portion provided with an integral stub.
In the rotor blade of a steam turbine disclosed in Japanese Patent No. 4058906, a distance between end surfaces of the integral shrouds (first coupling members) facing each other is set to be shorter than a distance between end surfaces of the integral stubs (second coupling members) facing each other. Thus, the integral shrouds come into contact with each other first, and then the integral stubs come into contact with each other as the rotation speed of the rotor increases. Thus, excessive stress is prevented from being generated at a coupling portion between the coupling members and the blade portion. The steam turbine having this configuration is regarded as having the rotor blade with improved reliability in terms of hardness and vibration, in an operation range from the start to a rated operation of the turbine.
In Japanese Patent Application Publication No. 2011-137424, an intermediate coupling member, provided to the rotor blade as a long blade, for improving vibration characteristics serves as a resistance in a path of steam flowing between the rotor blades, and thus degrades an aerodynamic performance. Thus, in the turbine rotor cascade disclosed in Japanese Patent Application Publication No. 2011-137424, the positional relationship between the rotor blades of the intermediate coupling members and the cross-sectional shape of the intermediate coupling member are optimized to reduce aerodynamic losses between the rotor blades.
The steam in the steam turbine is used for expansion work in each turbine stage, to have the energy reduced. Thus, in a stage on a more downstream side, the steam temperature is low and thus results in a steam path portion being a wet region. The turbine rotor blade rotating in the wet region might have a front edge portion, at the distal end of the blade involving high circumferential speed, eroded due to corrosion of droplets in the steam. In view of this, Japanese Patent Application Publication No. 2004-270023 discloses a method for treating devices subject to erosion by liquids and an anti-erosion coating alloy.
The method for treating devices subject to erosion by liquids disclosed in Japanese Patent Application Publication No. 2004-270023 includes applying a cobalt-based alloy, having a predetermined composition, by laser plating on the blade of the steam turbine as the device subject to erosion by liquids.
Japanese Patent Application Publication No. 2014-163371 discloses a technique of forming a coating on a coupling member (cover and tie boss) contact surface of a turbine rotor blade made of a titanium alloy by high-velocity flame spraying (HVOF spraying) in which chromium carbide (CrC) is sprayed with NiCr used as a binder. Thus, the high-velocity flame spraying in which a mixture is sprayed is disclosed in Japanese Patent Application Publication No. 2014-163371.